Integrable fiberoptic coupler and resulting devices and systems

ABSTRACT

A fiberoptic coupler capable of many functions is presented. The basic fiberoptic coupler has a first sleeve, a second sleeve, a first collimating GRIN or conventional lens, and a second collimating GRIN or conventional lens. The first sleeve holds end sections of two or more input optical fibers along the longitudinal axis of the sleeve. The second sleeve holds an end section of at least one output optical fiber. The end face of the second sleeve faces the first sleeve end face. The first collimating GRIN or conventional lens in front of the first sleeve end face collimates light signals from the input optical fibers and the second collimating GRIN or conventional lens in front of the second sleeve end face focusses light signals from at least one of the input optical fibers into the single output fiber, or at least one of the output optical fibers. With only one output fiber the coupler operates as a combiner. If more than one output fiber is held by the second sleeve, the input and output fibers can be arranged so that a light signal from one input fiber is sent to one output fiber. For added functionality, optical elements, such as isolators and wavelength-dependent filters, can be inserted between the first and second collimating lenses.

This is a Continuation of application Ser. No. 08/470,815, filed Jun. 6,1995, now abandoned, which is a CIP of 08/361,610, filed Dec. 21, 1994U.S. Pat. No. 5,555,330, the disclosure of which is incorporated byreference.

BACKGROUND OF THE INVENTION

The present invention is related to the field of optical fibertechnology and, more particularly, to fiberoptic couplers, especiallywavelength division multiplexed (WDM) couplers, and fiberoptic isolatordevices.

In strict fiberoptic terminology, fiberoptic "couplers" are deviceswhich transfer the light signals from a plurality of input fibers to aplurality of output fibers. "Combiners" are devices by which the lightsignals from more than one input fiber are combined into a single outputfiber. However, as explained below, the present invention is readilyadaptable to both types of devices. Hence the term, "coupler," as usedwith respect to the devices of the present invention is meant to beinterpreted broadly and to cover both terms.

A WDM coupler transfers input signals from a plurality of inputinformation channels to a plurality of output information channels inresponse to the wavelength of the input signals. A goal for any WDMcoupler is that the crosstalk between channels is zero, i.e., that anuntargeted output channel is effectively isolated from the signals on atargeted output channel so that none of the signals leak onto theuntargeted channel.

FIG. 1A is a representational diagram of a 2×2 WDM coupler found in theprior art. The cladding and core of a pair of optical fibers are fusedtogether to form a WDM coupler 10 enclosed by dotted lines. The coupler10 has two input fibers 11, 12 and two output fibers 13, 14. The firstinput fiber 11 carries signals of wavelength λ₁ and the second inputfiber 12 carries signals of wavelength λ₂. Ideally, only one of theoutput fibers, say output fiber 13, should carry the signals ofwavelength λ₁, while the other output fiber 14 should carry the signalsof wavelength λ₂, as shown in FIG. 1A. Crosstalk occurs if the λ₁signals appear on the output fiber 14 or the λ₂ signals appear on theoutput fiber 13.

An application of the fused coupler of FIG. 1A is a partially integratedWDM coupler and isolator device, which is described in U.S. Pat. No.5,082,343, issued Jan. 21, 1992 to D. G. Coult et al. The fused WDMcoupler is illustrated in FIG. 1B. The coupler 20 (again enclosed bydotted lines) has two input fibers 15, 16 and an output fiber 17. Thetwo input fibers 15 and 16 are fused together and signals from the fusedfibers are directed toward a collimator 18. A second collimator 19refocuses the collimated light signals from the collimator 18 into theoutput fiber 17. The collimators 18 and 19 are shown as standard lensesfor purposes of illustration. One input fiber 15 carries signals ofwavelength λ₁ ; the second input fiber 16 carries a signals ofwavelength λ₂. A wavelength selective element 21 between the collimators18 and 19 reflects the light of one of the wavelengths, say λ₁, andpasses the λ₂ wavelength light. Thus the output fiber 17 carries the λ₂signals.

The problem with this WDM coupler and isolator arrangement is thecrosstalk between the fibers 15 and 16 carrying the reflected λ₁signals. As explained below, the λ₁ signal should ideally be reflectedinto the input fiber 16 only. In reality, some of the λ₁ signal isreflected back into the input fiber 15; there is crosstalk. Besidescrosstalk, another problem is that the insertion losses and polarizationdependent losses of such couplers are high. Additionally, the device israther large, which has an adverse effect upon the reliability androbustness of the device. Sealing the device, for example, is more of aproblem with a large device. The large size also makes the devicedifficult to insert into various points of a fiberoptic network system,as may be desired. Still another disadvantage of the described device isthat other desirable components, such as a tap coupler for monitoringsignals through the device, must still be linked by fiber splicing. Thislowers the performance of the overall system and creates furtherreliability problems.

In contrast, the present invention avoids, or substantially mitigates,the problems of the fused coupler. A fiberoptic coupler according to thepresent invention has a much higher optical performance and is easilyintegrated with other optical elements to create integrated couplers andoptical isolators with advanced features and performance. Among thesecoupler and isolator devices is included an integrated WDM coupler andisolator device with high isolation between channels. The device is agreat improvement over the WDM coupler and isolator described in thepatent noted above.

Furthermore, these advanced couplers and isolators provide for advancedfiberoptic systems of higher performance, lower cost, and superiorreliability.

SUMMARY OF THE INVENTION

The present invention provides for a fiberoptic coupler which has afirst sleeve, a second sleeve, a first collimating GRIN or conventionallens, and a second collimating GRIN or conventional lens. The firstsleeve has an end face, a longitudinal axis and an aperture parallel tothe longitudinal axis through the end face. The aperture holds endsections of two or more input optical fibers. The second sleeve has anend face, a longitudinal axis and an aperture parallel to thelongitudinal axis through the end face with the aperture holding an endof at least one output optical fiber. The end face of the second sleevefaces the first sleeve end face. The first collimating GRIN orconventional lens in front of the first sleeve end face collimates lightsignals from the input optical fibers and the second collimating GRIN orconventional lens in front of the second sleeve end face focusses lightsignals from at least one of the input optical fibers into the singleoutput fiber, or at least one of the output optical fibers.

With an equivalent number of output optical fibers to input opticalfibers, the first sleeve holding the ends of the input optical fibersand the second sleeve holding the ends of the output optical fibers, thefirst and second collimating lenses of the coupler are arranged so thatlight signals from one input optical fiber passes into one outputoptical fiber. Light signals from another input optical fiber passesinto another output optical fiber, and so forth.

If the coupler is required for certain functions, other fiberopticelements, such as an optical isolator core or a long-pass filter, can beintegrated into the coupler between the optical path formed between thefirst and second collimating lenses. The long-pass filter has a cut-offwavelength above which light signals are passed and below which lightsignals are reflected. The filter is arranged with respect to the firstcollimating lens and the ends of the input optical fibers are arrangedwith respect to each other so that light from a first input fiber andreflected by the long-pass filter passes into the second input fiber andlight from the first input fiber and passed by the long-pass filterleaves the long-pass filter as collimated light.

By inserting both an optical isolator and a long-pass filter, thepresent invention also provides for an integrated WDM coupler andisolator highly suited for combination with fiberoptic amplifiers andpumping lasers for the fiberoptic amplifiers. If an optical tap isdesired, a planar grating may be added in the optical path between thetwo collimating lenses. The grating deflects a small portion of thelight away from the optical path to a photodetector circuit which isused to monitor the intensity of light in the optical path.

With the present invention, a laser diode may also be integrated intothe coupler. The laser directs its output against the long-pass filter,which reflects the laser output onto the optical path. This arrangementnot only integrates the pumping laser into an integrated package, butalso increases the performance over the prior art.

The present invention also provides for an advanced system fortransmitting light signals from a plurality of sources onto an opticalfiber of a WDM optical fiber network. The system has a first combinerwhich has several input terminals and an output terminal connected tothe optical fiber. The system also has a plurality of laser diodes, eachdiode generating light signals at a predetermined wavelength for one ofthe sources, and a plurality of couplers integrated with opticalisolator elements, in accordance with the present invention. Eachintegrated coupler has two or more input optical fibers and at least oneoutput optical fiber, each input optical fiber connected to a laserdiode and the output optical fiber connected to one of the firstcombiner input terminals. Each integrated coupler passes light signalsfrom the input optical fibers to the output optical fiber and blockslight signals from the output optical fiber to the input optical fibers.

Finally, the present invention provides for an advanced receiving systemof light signals from a plurality of sources on a transmission opticalfiber of a WDM optical fiber network. The system is connected betweenthe transmission optical fiber and a plurality of receivers. The systemhas a plurality of couplers, according to the present invention. Eachcoupler is integrated with a long-pass filter element. The first couplerof the receiving system has its first optical fiber connected to thetransmission optical fiber. The receivers and the remaining couplers areconnected to each other and the first coupler. The predetermined cut-offwavelength of each long-pass filter of the couplers is selected in amanner so that each receiver receives light signals at a particularwavelength from a second or third optical fiber from one of thecouplers. Thus the couplers can be arranged in a cascade arrangement, ahybrid arrangement or a combination of both to meet the particularspecifications of the receiving system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a representational diagram of a fused fiber, WDM coupleraccording to the prior art; FIG. 1B is a variation of the WDM coupler ofFIG. 1A;

FIG. 2 is a cross-sectional view of a coupler according to the presentinvention;

FIG. 3 is a detailed view of the ends of the input optical fibers of thecoupler of FIG. 2;

FIG. 4 is a representational illustration of the travel of the lightsignals between the input and output optical fibers of the coupler ofFIG. 2;

FIG. 5A is a tracing of light rays passing through a half-pitch GRINlens; FIG. 5B is a tracing of light rays passing through twoconventional lenses designed to operate like a half-pitch GRIN lens;

FIGS. 6A-6C illustrates some of the steps of manufacturing the couplerof FIG. 2;

FIG. 7 is a cross-sectional view of another embodiment of the coupler,here a combiner, according to the present invention;

FIG. 8 is a cross-sectional view of another embodiment of the coupler,including an optical isolator subassembly, according to the presentinvention;

FIGS. 9A-9B illustrates some of the steps of manufacturing the couplerof FIG. 8;

FIG. 10 is a cross-sectional view of another embodiment of the coupler,including a high-pass filter, according to the present invention;

FIG. 11 illustrates an early step of manufacturing the coupler of FIG.10;

FIG. 12 is a cross-sectional view of a variation of the coupler of FIG.10;

FIG. 13 is a cross-sectional diagram of one embodiment of a wavelengthdivision multiplexed coupler and isolator in a forward pumpingarrangement according to the present invention

FIG. 14 is a diagram of one embodiment of a wavelength divisionmultiplexed coupler and isolator in a forward pumping arrangementaccording to the present invention;

FIG. 15 is a diagram of the wavelength division multiplexed coupler andisolator of FIG. 14 in a backward pumping arrangement according to thepresent invention;

FIG. 16 is a representational diagram of another embodiment of awavelength division multiplexed coupler and isolator in a forwardpumping arrangement according to the present invention;

FIG. 17 illustrates how the integrated wavelength division multiplexedcoupler and isolator of FIGS. 13-15 can be used in a double pumpingarrangement according to the present invention;

FIG. 18 is an integrated WDM coupler and isolator according to thepresent, invention in a fiberoptic network;

FIG. 19 is a representational block diagram of a WDM network accordingto the prior art;

FIG. 20 is a representational block diagram of an advanced transmissionsystem according to the present invention for a WDM network;

FIG. 21 is a representational block diagram of another transmissionsystem according to the present invention for a WDM network:

FIG. 22 is a representational block diagram of an advanced receiversystem using the coupler of FIG. 2 according to the present inventionfor a WDM network;

FIG. 23 is a graph plotting reflectivity versus wavelength of thecouplers in FIG. 22;

FIG. 24 is an illustration of one of the couplers of FIG. 22 and someassociated optical parameters; and

FIG. 25 is a representational block diagram of another advanced receiversystem according to the present invention for a WDM network.

In passing, it should be noted that the same reference numerals are usedin the drawings where the element or function of an element has notchanged to further an understanding of the present invention in itsvarious aspects.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT(S)

Basic Intearable Fiberoptic Coupler

FIG. 2 is a cross-sectional view of a fiberoptic coupler according tothe present invention. The coupler combines the end sections of twoinput optical fibers 30 and 31, which are not fused together, in anopening 37 through the longitudinal axis of a glass sleeve 33.Similarly, the end sections of output fibers 32 and 39 are held in anopening 38 through the longitudinal axis of a glass sleeve 36. Thesleeve 33 has a slant-angled face which is in close proximity with areciprocally slanted face of a quarter-pitch GRIN (GRaded INdex) lens34. Similarly the sleeve 36 has a slant-angled face which is closeproximity with a reciprocally slanted face of a quarter-pitch GRIN lens35.

The end sections of the input optical fibers 30 and 31, and outputoptical fibers 32 and 39 are formed by unjacketed optical fibers. Thecore and cladding of each fiber are exposed, and the exposed claddingand core may or may not be tapered. FIG. 3 is a detailed cross-sectionalend view of the opening 37 in the sleeve 33. In this example, the endsections of the fibers 30 and 31 are untapered. Hence thecross-sectional diameter of each fiber is 125μ, the typical claddingdiameter of a single mode fiber. The diameter of the opening 37 is 250μto snugly accommodate the two fibers 30 and 31. Similarly thecross-sectional diameter of the opening 38 in the sleeve 36 is 250μ toaccommodate the untapered end sections of the output fibers 32 and 39.

Light from the input fibers 30 and 31 is collimated by the GRIN lens 34.The collimated light is refocussed by the second GRIN lens 35 at theends of the output fibers 32 and 39. However, the coupler does not mixthe light signals from the input fibers 30 and 31 into the output fibers32 and 39. FIG. 4 represents the transmission of light signals from theinput fibers to the output fibers by solid circles and "+"'s. Light withwavelength λ₁ from the input fiber 30 is received by the output fiber 32and light with wavelength λ₂ from the input fiber 31 is received by theoutput fiber 39.

Functionally, the two quarter-pitch GRIN lenses 34 and 35 may beconsidered as a half-pitch GRIN lens which has been split into two equalparts. FIG. 5A illustrates the action of a half-pitch GRIN lens 80. Adotted line 83 illustrates where the half-pitch GRIN lens 80 might beseparated into two quarter-pitch GRIN lenses 81 and 82. The GRIN lens 80has a longitudinal axis 86. A point source of light A at one surface ofthe lens 80 on the axis 86 appears as a point A' at the other surface ofthe lens 80 on the axis 86. This is shown by a tracing of rays frompoint A to point A'. A point B at one surface of the lens 80 butslightly off the longitudinal axis 86 appears as a point B' at the othersurface of the lens 80 equally removed from, but on the other side of,the longitudinal axis 86.

As illustrated in FIG. 2, the input fibers 30 and 31 fit snugly into thecentral opening 37 through the sleeve 33. The cladding of the fibers 30and 31 maintain a distance between the cores of the two fibers.Likewise, the output fibers 32 and 39 fit snugly into the centralopening 38 through the sleeve 36 and the cladding of the fibers 32 and39 maintain a distance between the cores of these two fibers. Asexplained above, the two quarter-pitch GRIN lenses 34 and 35, thoughseparated, operationally form one half-pitch GRIN lens. The ends of theinput fibers 30, 31 and the output fibers 32, 39 are explained withreference to the longitudinal axis of the combined GRIN lenses 34 and35.

The ends of the input fibers 30 and 31 are arranged with respect to oneend surface of the GRIN lens 34 so that each end, specifically the fibercore, is slightly removed from the longitudinal axis. Correspondingly,the ends of the output fibers 32 and 39 are also arranged with respectto the other end surface of the GRIN lens 35 so that each fiber end isslightly removed from the longitudinal axis and opposite to an inputfiber. The result is that light from the core of one input fiber 30(31)is received by the core of an output fiber opposite the input fiber32(39) with respect to the longitudinal axis of the combinedquarter-pitch GRIN lenses 34 and 35. Alternatively, one input fiber andone output fiber may be located along the longitudinal axis with theother input fiber and output fiber located opposite each other withrespect to the longitudinal axis.

It should be noted that the explanation above of the positions of thefiber ends with respect to the longitudinal axis, as called for by thetheoretical operation of the GRIN lens, is an idealization. Empirically,it has been found that fine adjustments may still be required to achievemaximum performance of the WDM coupler.

FIGS. 6A-6C illustrate some of the steps useful in the manufacture ofthe coupler shown in FIG. 2. The GRIN lenses used in the coupler arequarter-pitch in theory, but in practice it has been found that 0.23pitch offers better collimating performance. While standard lenses couldalso be used as collimators, it has been found that GRIN lenses providebetter performance, easier manufacturing and greater durability.

As shown in FIG. 6A, the back face of the GRIN lens 34 is polished at anangle, shown here at an exaggerated angle. Typically, the polish angleis 8°-12° from a flat surface perpendicular to the longitudinal axis ofthe GRIN lens 34. The ends of two input optical fibers 30 and 31 havetheir protective jackets removed, and the core and cladding at the endsections of these fibers may be tapered or left untapered. To taper thefiber end sections, the fiber ends are repeatedly dipped into a bufferedHF solution. The two ends of the fibers are then inserted into thequartz glass sleeve 33 which has the central opening 37 sufficientlylarge to accept the end sections of the two fibers 30 and 31. The endsof the two fibers extend beyond the end of the sleeve 33 and are cutflush against the forward face of the sleeve. The forward face of thesleeve 33 is then polished at the same angle of the back face of theGRIN lens 34. Antireflection coatings are deposited on the forward faceof the sleeve 33 and the back face of the GRIN lens 34. The front faceof the glass sleeve 33 and the back face of the GRIN lens 34 are thenbrought together in close proximity with the angle of their faces inparallel and reciprocal relationship. Separation distance is 5 to 10 μm.The sleeve 33 and GRIN lens 34 are held in place by an UV-curved epoxy49 or a solder.

As shown in FIG. 6B, the sleeve 33 is placed in a quartz cylinder 46which holds the ends of the optical fibers 30 and 31, the sleeve 33 andthe GRIN lens 34 centered in a cylindrical housing 47 of stainlesssteel. The housing 47 forms the outer covering of a collimatorsubassembly 50. Epoxy 48, such as Model 4481 from Electro-Lite Company,Danbury, Conn., holds the subassembly 50 in place.

A second collimator subassembly 51 is similarly formed by thequarter-pitch GRIN lens 35 which has its back face similarlyangle-polished and in reciprocal relationship to the angle-polishedfront face of the quartz sleeve 36 which holds the end of the outputoptical fiber 32. This is shown in FIG. 6C. A quartz cylinder 44 holdsthe GRIN lens 35 and the optical fibers 32 and 39 in place in acylindrical subassembly housing 45. As shown in FIG. 6C, the front facesof the two GRIN lens 34 and 35 face each other to form an optical path.The subassemblies 50 and 51 form a coupler. That is, light signals onthe input fibers 30 and 31 pass to the output fibers 32 and 39.

More than two optical fibers may be inserted into the central openings37, 38 of the sleeves 33, 36. Of course, the diameters of the openings37, 38 must be enlarged to accommodate the additional fibers. Successfulresults have been obtained with 4×4 couplers. A 5×5 coupler isconceivable with one fiber along the longitudinal axis of the GRINlenses and the remaining four fibers spaced around the central fiber.

Thus with a half-pitch GRIN lens or two separated quarter-pitch GRINlens, the coupler can transmit light signals from an input fiber to anoutput fiber. Another input fiber transmits its signals to anotheroutput fiber, and so on.

The described coupler can operate in a WDM network and could beconsidered as a WDM coupler. However, the couplers might best beconsidered as a simple coupler since an input fiber is coupled to anoutput fiber by the arrangement of the fibers with respect to each other(and any intervening optical element(s), as described below), ratherthan as a function of a signal's wavelength. The coupler is adaptable tomany functions. If a coupler is needed to pass signals between input andoutput fibers, a half-pitch GRIN lens with input and output fibersarranged as described. If additional functions are required to beintegrated into the coupler, elements can be easily inserted into theoptical path between quarter-pitch GRIN lenses.

Conventional homogeneous lenses might also be used in place of the GRINlenses in the coupler, though GRIN lenses are believed to be superior inthe balance of factors, such as size, cost, performance and reliabilityconsiderations. Conventional collimating lenses, including homogeneousand aspheric lenses, might be used in place of the quarter-pitch GRINlenses. Conventional lenses might also be used in place of a half-pitchGRIN lens. FIG. 5B illustrates two conventional lenses 90 and 91 whichare designed to operate, and to the same scale, as the half-pitch GRINlens 80 in FIG. 5A. The numerical apertures for the light beams, thedistance of the off-axis points D and D' from the central axis 96, andthe diameters of the collimated light are also equal.

FIG. 7 is a cross-sectional view of another embodiment of the fiberopticcoupler according to the present invention. In this case the coupler isformed as a fiberoptic combiner by which a plurality of input fibers canpass their light signals to a single output fiber.

As described previously with respect to the coupler of FIG. 2, the endsections of two input optical fibers 70 and 71, which are not fusedtogether, are inserted into a central opening 77 through thelongitudinal axis of a glass sleeve 73. Instead of two output fibers,the end section of a single output fiber 72 is held in a central opening78 through the longitudinal axis of a glass sleeve 76. The sleeve 73 hasa slant-angled front face which is in close proximity with areciprocally slanted back face of a quarter-pitch GRIN (GRaded INdex)lens 74. Similarly the sleeve 76 has a slant-angled front face which isin close proximity with a reciprocally slanted back face of aquarter-pitch GRIN lens 75. As before, the end sections of the inputoptical fibers 70 and 71, and output optical fiber 72 are formed byunjacketed sections of optical fibers to expose the core and cladding ofeach fiber. The exposed cladding and core may or may not be tapered.

Since the described coupler is a combiner in which light signals fromthe two input fibers 70 and 71 are combined into the output fiber 72, itis desirable that the output fiber 72 receive as much of the light fromboth input fibers as possible to reduce fractional loss. The pitch ofthe GRIN lens 75 is slightly more, or less, than a functionalquarter-pitch so that the light from each of the input fibers 70 and 71is not refocussed to a point. The light from each of the input fibers isslightly defocussed so that the end of the output fiber 72 receives atleast a portion of the light.

If the difference between the wavelengths on the two input fibers 70 and71 is sufficiently large, the fractional loss of the light from each ofthe input fibers to the output fiber 72 is acceptable. On the otherhand, if the difference between the wavelengths is not sufficientlylarge, then the end of the output fiber 72 is formed with a properlyenlarged core to increase the transfer of light from each of the inputfibers 70 and 71 to the output fiber and to keep the fractional losslow. Suitable fibers with properly enlarged cores includethermally-expanded expanded core optical fibers from Sumitomo OsakaCement, Ltd. of Tokyo, Japan.

If additional functions are required to be integrated into the coupler,fiberoptic elements can be easily inserted into the optical path betweenthe quarter-pitch GRIN lenses of the couplers illustrated in FIGS. 2 and7.

Coupler Integrated with Optical Isolator

In the coupler embodiment in FIG. 8, the elements of an optical isolatorcore are inserted between the GRIN lenses 34 and 35 of FIG. 2 so theresulting coupler has isolation functions. The optical isolator coreensures that light in the forward direction is sent through the coupler;light in the backward direction is effectively blocked.

Between the two quarter-pitch GRIN lenses 34 and 35, an optical isolatorcore is formed by two wedge-shaped birefringent crystals 40 and 42located on either side of a Faraday rotator 41, which requires amagnetic field for its operation. An annular-shaped magnet 43 holds thecrystals 40 and 42, and the rotator 41.

Light from either input fiber 30 or 31 is collimated by the GRIN lens34. After passing through the optical isolator core, the collimatedlight is recollimated or focused at the end of the output fibers 32 or39 by the GRIN lens 35. Conversely, one might think that light from theoutput fibers 32 or 39 might be collimated by the GRIN lens 35 andrefocussed into the ends of the input fibers 30 and 31. However, theaction of the optical isolator core allows light to travel in only onedirection and blocks light in the opposite direction. An explanation ofthe action of the optical isolator core is found in U.S. Pat. No.5,208,876, entitled, "Optical Isolator," which issued May 4, 1993 and isassigned to the present assignee.

FIGS. 9A and 9B illustrate the construction of an isolator core assembly52 between the collimator subassemblies 50 and 51. The steps in FIGS. 9Aand 9B continue the manufacturing steps of FIGS. 6A-6C. The opticalisolator subassembly 52 has the two birefringent crystal polarizers 40and 42 on either side of the Faraday rotator 41. These three elementsare held in place in a quartz glass cylinder 49. The cylinder 49 engagesthe portion of the GRIN lens 35 which protrudes cylinder 44. Around thecylindrical holder 49 is the annular magnet 43 which functions with theFaraday rotator 41. Materials which may be used for the rotator 41include garnet doped with impurities or, YIG, and TGG.

These materials heretofore used as Faraday rotators in optical isolatorsoperate very well at wavelengths from 1200 to 1600 nm. However, opticalisolators are sometimes used with lasers operating at shorterwavelengths, specifically, 980 and 1017 nm. CdMnTe (Cadmium manganesetellurium), or Hg-doped CdMnTe, has a large Verdet constant whichpermits the Faraday rotator to be small enough to fit into theintegrated WDM coupler and isolator, as contemplated by the presentinvention and described below, and to effectively operate at suchshorter wavelengths. CdMnTe (more precisely, Cd_(1-x) Mn_(x) Te) orHg-doped CdMnTe may also be used as Faraday rotators for operations atlight wavelengths from 1200 to 1600 nm, and may be used on Faradayrotators in standard optical isolators.

FIG. 9B illustrates the completed coupler with the two collimatorsubassemblies 50 and 51, and the optical isolator subassembly 52. Thecompleted device can be integrated in a commercially available 14-pinbutterfly package 53, approximately 21 mm long and 13 mm wide, forminiaturization. The package is hermetically sealed by a combination oflaser and solder welding for increased integrity and reliability.

More detailed information on the manufacture of GRIN lens/optical fibersubassemblies, and isolator core subassemblies discussed below, may befound in U.S. Pat. No. 5,208,876 described above.

FIG. 10 illustrates the insertion of the optical isolator subassembly ofFIG. 8 into the coupler of FIG. 7. It is readily evident that the FIG.10 coupler now has isolation functions. Light signals can pass fromeither input fiber 70 or 71 to the output fiber 72, but light is blockedfrom the output fiber 72 to the input fibers 70 and 71.

Coupler Intearated with Wavelength-Dependent Filter

In the coupler with isolation functions, an optical isolator core isinserted between the GRIN lenses of the coupler. If awavelength-responsive element is inserted between the two GRIN lenses, aWDM coupler is created.

For example, FIG. 11 illustrates the coupler of FIG. 7 with a long-passfilter 44 between the two quarter-pitch GRIN lenses 74 and 75. Thefilter 44 has a cutoff wavelength, i.e., a wavelength above which thefilter 44 passes light through and below which the filter reflects thelight back.

Light from either fiber 70 or 71 is collimated by the GRIN lens 74. Thecollimated light which is not reflected by the long-pass filter 44 isrecollimated or focused at the end of the output fiber 72 by the GRINlens 75. Assuming that input light signals of wavelength λ travel on thefiber 70, the light signals are either reflected back into the fiber 71or pass through to the output fiber 72 depending upon whether λ is lessor greater than the cutoff wavelength of the filter 44. The ends of thetwo fibers 70 and 71 are arranged with respect to the longitudinal axisof the GRIN lens 74 so that light from the fiber 70 which is reflectedback by the filter 44 is refocussed by the GRIN lens 74 at the end ofthe fiber 71. The reflection relationship is reciprocal between the twofibers 70 and 71. A WDM coupler is, in fact, formed by the ends of thefibers 70, 71, the quarter-pitch GRIN lens 74 and the filter 44 sinceincoming light is transferred responsive to the wavelength of the light.

The operation of the WDM coupler is understood as follows: The ends ofthe input fibers 70 and 71 are arranged with respect to one end surfaceof the GRIN lens 74 so that the end each fiber, specifically the fibercore, is slightly removed from the longitudinal axis of GRIN lens 74. Asexplained above, two quarter-pitch GRIN lenses operationally form onehalf-pitch GRIN lens. Thus the light signals from the fiber 70 intravelling through the quarter-pitch GRIN lens 74 and being reflectedback by the long-pass filter 44 through the GRIN lens 74 again passthough, in effect, a half-pitch GRIN lens. Since the light emanates fromthe end of the fiber 70 which is slightly displaced from thelongitudinal axis of the lens 74, the reflected light is refocussed atthe end of the fiber 71 which is slightly displaced in the oppositedirection from the longitudinal axis. By symmetry, it is easy to seethat light from the fiber 71 which is reflected by the filter 44 isrefocussed at the end of the fiber 70. Light passing through the filter44 is refocussed by the second quarter-pitch GRIN lens 35 at the end ofthe fiber 72. In this case, the two quarter-pitch GRIN lenses 74 and 75act as a half-pitch GRIN lens.

Previously described manufacturing steps in FIGS. 6A-6C may be used inthe manufacture of the WDM coupler. Prior to the step illustrated inFIG. 6A, the long-pass filter 44, typically a dichroic mirror filterplate, is attached to the flat front of the quarter-pitch GRIN lens 74,as shown in FIG. 12. Alternatively, dichroic filter material can bedeposited directly onto the GRIN lens surface. In other words, ananti-reflection coating is deposited on the exposed face of the filter44. The filter 44 can also be mounted separately from the GRIN lens 74,and both surfaces of the filter 44 are coated with anti-reflectionmaterial. This separate mounting is considered less desirable due to thesimplicity of the previous alternatives.

Another embodiment of the WDM coupler is illustrated in FIG. 13. In thiscase the long-pass filter 44 is inserted between the GRIN lenses 34 and35 of the coupler in FIG. 2. The coupler has two forward fibers 30, 31and two output fibers 32, 39. Operationally light having wavelengthsabove the cut-off wavelength of the filter 44 pass from the forwardfibers 30, 31 to the output fibers 32, 39 as illustrated by FIG. 4. Forwavelengths below the cut-off wavelength, light from the fiber 30 isreflected by the filter 44 back into the fiber 31, and vice versa.

Furthermore, more than two optical fibers may be inserted into thecentral openings 37, 38 of the sleeves 33, 36. Of course, the diametersof the openings 37, 38 must be enlarged to accommodate the additionalfibers. Successful results have been obtained with 4×4 WDM couplers.

Integrated WDM Coupler and Isolator

The high isolation performance of the WDM coupler leads to a WDM couplerwhich has the same functions of the WDM coupler of FIG. 1B. The priorart WDM coupler of FIG. 1B and the patent cited with respect to thedrawing were directed toward applications with fiberoptic amplifiers.Fiberoptic amplifiers boost message signals of one wavelength from thepump signals of another wavelength. These fiberoptic amplifiers,especially erbium-doped fiber amplifiers, are increasingly being used inhigh-speed fiberoptic transmission links and networks. These types ofamplifiers are readily insertable into various points of a network toprovide repeater functions, for example, to boost optical signalstraveling through many kilometers of optical fibers.

The fiberoptic amplifiers are coupled to lasers which supply the pumpsignals by wavelength division multiplexed couplers. Since the lasersare susceptible to noise, isolators are also coupled to theamplifier/laser systems to block noise and spurious signals whichdenigrate the performance of the amplifier. These devices allow a pumplaser to be effectively coupled to the fiberoptic amplifier so that amessage signal through the fiberoptic amplifier is amplified from theenergy supplied from the laser signal.

Thus, if an optical isolator subassembly is also inserted with ahigh-pass filter between the GRIN lenses of the FIG. 11 coupler, anintegrated WDM coupler and isolator device of superior performance isproduced. The input fiber 70 carries light of wavelength λ₂ and theinput fiber 71 carries light of a shorter wavelength λ₁. Between the twoGRIN lens is a long-pass filter 44, a dichroic filter, which has acutoff wavelength is set below the wavelength λ₂ on the input fiber 70and above the wavelength λ₁ on the input fiber 71.

In the manner explained above, the ends of the input fibers 70 and 71are arranged with respect to the longitudinal axis of the GRIN lens 74so that light from the input fiber 71 reflected back by the dichroicfilter 44 is refocussed by the GRIN lens 74 at the end of the inputfiber 70. The light from the input fiber 71 is sent to the input fiber70 and the light from the input fiber 70 is passed forward through thefilter 44 as collimated light. The GRIN lens 75 recollimates the lightfrom the input fiber 70 by refocussing the light to the end of theoutput fiber 72.

The resulting integrated coupler and isolator can operate effectivelywith fiberoptic amplifiers energized by pump lasers operating atwavelengths between 900 to 1200 nm. For example, message signal at 1550nm wavelength can use shorter wavelengths from the pump laser to drivethe coupled fiberoptic amplifier. If the pump laser generates an outputat 1480 nm, say, then standard Faraday rotator materials can be used inthe optical isolator subassembly 29 (see FIG. 14). If a pump laser at980 or 1017 nm is desired, a Faraday rotator of CdMnTe should be used.Of course, CdMnTe or Hg-doped CdMnTe may also be used for a Faradayrotator in a separate isolator device as described in U.S. Pat. No.5,208,876 noted above.

The WDM coupler and isolator is compact for convenient insertion intofiberoptic networks. The complete device can be integrated into acommercially available 14-pin butterfly package, approximately 21 mmlong and 13 mm wide, for miniaturization. The package is hermeticallysealed by a combination of laser and solder welding for increasedintegrity and reliability.

The described wavelength division multiplexer coupler has a highperformance. Insertion loss has been found to be 0.2 dB, compared to 0.5to 1.0 dB for fused fiber WDM couplers, and the polarization dependentlosses have been found to be 0.01 dB, compared to 0.1 db for the fusedfiber couplers. Crosstalk is very low; isolation losses exceed 30 dBcompared to 18 dB for the fused fiber couplers. For the couplers inFIGS. 1B and 14, the isolation loss is defined as the ratio of theintensity of light from one input fiber over the intensity of lightreflected back into the other input fiber.

The coupler of the present invention allows for WDM coupler and isolatordevices which retain excellent performance with other integrated opticalelements. FIG. 15 diagrammatically illustrates a WDM coupler andisolator in which an optical tap device has been added, according to thepresent invention. The device has a housing 99 which holds collimatorsubassemblies 110 and 111. The subassembly 110 holds the ends of twooptical fibers 100 and 101 and the subassembly 111 holds the end of theoptical fiber 102. Each subassembly holds the end of the fiber(s) with acollimating GRIN lens. The subassemblies and collimating GRIN lenses arearranged to form an optical path between each other.

In front of the subassembly 110 and attached to the front of thesubassembly is a long-pass filter 114, a dichroic mirror filter 114. Infront of the filter 114 there is an optical tap in the form of a beamsplitter 117 which deflects a small part of the light away from theoptical path between the two subassemblies 110 and 111. An optionalbandpass filter 118 is the next element in the optical path, which hasan optical isolator core subassembly 104 directly in front of thesubassembly 111. The arrow associated with the subassembly 104 indicatesthe direction along which the subassembly 104 permits light signals topass. Signals in the opposite direction are blocked.

The integrated wavelength division multiplexed coupler and isolatorshown in FIG. 15 operates with a fiberoptic amplifier, typically anoptical fiber doped with a rare earth metal, such as erbium (Er),praseodymium (Pr), neodymium (Nd), etc., and a pump laser. The opticalfiber 101 carries output signals from the pump laser (not shown in thedrawing) into the coupler and isolator device. The second optical fiber100 carries amplified information, i.e., message signals from the rareearth metal doped fiber (also not shown) into the multiplexed couplerand isolator and laser pump signals from the laser and the multiplexedcoupler and isolator device.

The amplified light signals from the optical fiber 100 are collimated bythe subassembly 110 which directs the light at the long-pass filter 114.At the same time, the laser pump generates light signals at a wavelengthbelow the wavelength of the message signal on the optical fiber 100.After the light from the optical fiber 100 is collimated by thesubassembly 110, the light is directed against the long-pass filter 114.The long-pass filter 114 has a cutoff wavelength set below thewavelength of the message signal and above the laser pump wavelength.Hence, the laser light from the optical fiber 101 is reflected back bythe long-pass filter 114 into the optical fiber 100 to the fiberopticamplifier. The amplifier, which operates independently of the directionof the light passing through, thus amplifies the message signals passinginto the wavelength division multiplexed coupler and isolator.

Since the wavelength of the message signal is above the cutoffwavelength of the long-pass filter 114, the signal passes through to thebeam splitter 117. The beam splitter in this embodiment is a simpleplanar grating which directs most of the light, approximately 97% of thelight, along the optical path between the subassemblies 110 and 111.

The light along the optical path passes through an optional bandpassfilter 118 which filters out the light signals at frequencies other thanthat of the message signals. The bandpass filter 118 may be left out ifthe message signal is sufficiently "clean" to pass to the output fiber102. It has been found that a bandpass filter increases the insertionloss of the wavelength division multiplexed coupler and isolatorslightly, approximately 0.2 dB.

The filtered light then passes into the isolator core subassembly 104,which is formed by two wedge-shaped birefringent crystal polarizers oneither side of a Faraday rotator. The subassembly 104 blocks any lighttravelling along the optical path from the collimator subassembly 111toward the first collimator subassembly 110. From the isolatorsubassembly 104, the light is then sent into the subassembly 111 whichrecollimates the light into a point focused at the end of the opticalfiber 102.

The grating 117 also deflects a small portion of the light away from theoptical path between the subassemblies 110 and 111 toward aphotodetector circuit 115 in the form of an integrated circuit having aphotodiode. The photodetector circuit 115 is responsive to the intensityof the light in the optical path. Hence, the photodetector circuit 115monitors the amount of light from the subassembly 110 to the subassembly111. This provides a simple monitoring of the operations of thefiberoptic amplifier on the fiber 100 and the laser pump on the fiber101.

The integrated coupler and isolator device of FIGS. 14 and 15 is aso-called "forward" pumping arrangement with the fiberoptic amplifier,which is placed ahead of the device. The device can also be rearrangedso that it can be in a "backward" pumping arrangement with thefiberoptic amplifier, as shown in FIG. 16. In this case the fiberopticamplifier is connected below or downstream of the integrated coupler andisolator. A message signal is sent from an optic fiber 120 to theintegrated coupler and isolator device in a housing 130. The end of thefiber 120 is held by a collimator subassembly 124, as described above,to which is attached a core isolator subassembly 129. At the end of thesubassembly 129 is a long-pass filter 134 and a grating beam deflector137. As described previously, the deflector 137 partially deflects someof the light from the optical path toward a photodetector circuit 135which monitors the strength of the optical signals through theintegrated device. The undeflected light is received by a collimatorsubassembly 125 which holds the spliced ends of the optical fibers 121and 122.

Operationally the optical fiber 121 is connected to a pump laser whichsends its output into the integrated coupler and isolator. After beingcollimated by the subassembly 125 the laser light is sent through thebeam deflector 137 and reflected back by the long-pass filter 134. Atthe same time the message signals are sent to the integrated coupler andisolator from the optical fiber 120. After passage through thesubassembly 124, the message signals combined with the reflected lasersignals travel along the optical path toward the subassembly 125. Thesecombined signals are partially deflected toward the photodetectorcircuit 135 for a monitor of the message signals and the laser signals.

The photodetector circuit in integrated coupler and isolator can bearranged with respect to the beam deflector anywhere in the optical pathbetween the collimator subassemblies. However, if the long-pass filter134 is placed against the end of the subassembly 125, the photodetector135 only monitors the strength of the unamplified message signals andnot the pump laser signal. To avoid this, the photodetector circuit inintegrated coupler and isolator can be arranged with respect to the beamdeflector so that the light monitored has an intensity related to thepower of the message signal laser. In the forward pump arrangement ofFIG. 14, the monitored light is the light from the rare earth metaldoped fiber which is boosted by the pump laser output. In the backwardpump arrangement of FIG. 15, the light directly from the input messagesignal and the pump laser signal is monitored.

Of course, the integrated coupler and isolator devices can also be usedin combination with each other. FIG. 18 illustrates a combined forwardand backward pumping arrangement for an erbium-doped fiberopticamplifier between the two integrated devices.

The embodiments of the present invention shown in FIGS. 14-16 have thepump laser delivering its signal to the device through an optical fiber.Despite the improved performance of the wavelength division multiplexedcouplers described here, the combination of optical fibers inherentlycauses some insertion loss and, of significance in recent years, somepolarization dependent losses. That is, depending upon the state ofpolarization of a signal in one of the spliced optical fibers, the lossin transmission to the other fiber is dependent upon the polarization ofthe light signal. This is unacceptable if the state of polarizationvaries in the information channel.

The embodiment of the present invention shown in FIG. 17 eliminates thisproblem. The wavelength division multiplexer and coupler has a metallic(suitable materials include Kovar and Invar, or stainless steel) packagehousing 150 with a first collimator subassembly 144 and a secondcollimator subassembly 145. Each subassembly 144, 145 holds the end ofone optical fiber 141, 142 respectively. Between the two facing ends ofthese subassemblies 144 and 145 which define an optical path betweenthem are a beam splitter 157, a long-pass filter 154, and the optionalbandpass filter 158 and an optical isolator subassembly 149. Theseelements are the same as previously described. There is also aphotodetector circuit 155 which receives the partially deflected lightfrom the optical path by the beam splitter 157, a planar grating.

Instead of an optical fiber from a pump laser, the housing 150 holds alaser subassembly having a laser diode 166 and a collimator 167. Lightfrom the laser diode 166 is collimated by the collimator 167, in thiscase, an aspheric lens. The light passes through a second opticalisolator core subassembly 168 and is directed against the surface of thedichroic long-pass filter 154. The dichroic long-pass filter 154 isarranged such that the light from the laser is directed in the reversedirection of the optical path toward the deflector 157 into thecollimator subassembly 144 and the input fiber 141. This permits thefiber amplifier connected to the optical fiber 141 to be pumped toamplify the information signal into the device.

A particular adaptation of the laser diode 166, collimator 167 andisolator subassembly 168 is described in a co-pending patentapplication, U.S. application Ser. No. 08/361,604, entitled,"MINIATURIZED LASER DIODE ASSEMBLY", filed Dec. 21, 1994 and assigned tothe present assignee. This adaptation is very compact and highly suitedfor insertion into the housing 150.

From FIG. 18 it is evident how an integrated WDM coupler and isolatoraccording to the present invention fits into a fiberoptic network. Thedescription below illustrates how the simpler forms of the coupler ofthe present invention, i.e., those integrated with isolator or filterelements only, not only fit into a WDM fiberoptic network but also yieldadvanced network systems of superior performance, lower cost and greaterreliability.

WDM Network Systems

In modern fiberoptic networks many individual sources of light signalsin one location are sent over a single optical fiber and thendistributed to many individual receivers in another location. In a WDMnetwork, the wavelengths of the light signals are used to discriminatebetween information channels in the network to connect an individualsource with an individual receiver.

FIG. 19 is a representative diagram of a conventional fiberoptic WDMnetwork in which a multiple source transmitter system 210 sends opticalsignals over a single optical fiber 220 to a receiver system 212. Theparticular wavelength of the light signals from each source is used todirect the optical signals from the source to the desired receiver. Anamplifier 211 is connected between sections of the fiber 220 torepresent one or more erbium-doped fiber amplifiers which maintain thestrength of the signals over the length of the fiber 220, typically asingle-mode fiber. In fact, single mode optical fibers are used in mostapplications today. Therefore, in the following description, an opticalfiber should be assumed to be a single mode fiber unless stateddifferently.

The transmitter system 210 has N sources of optical signals, each ofwhich is generated by a narrow linewidth laser 214 connected to an N×1optical combiner 216 through an optical isolator 215. Each laser 214 isconnected to a power supply 213, which is stable to keep the wavelengthof each laser 214 from varying from a predetermined wavelength. Thewavelength of each laser 214 is different from the wavelengths of theother diodes 214. The laser 214 is also connected to communicationelectronic circuitry (not shown) which controls the operation of thelaser 214. The isolator 215 also maintains each laser 214 at itspredetermined wavelength by blocking reflections and spurious signalsfrom the combiner 216 to the laser 214. The optical signals from the Nlaser sources are combined by the N×1 combiner 216 and sent through thefiber 220.

Conversely, the receiver system 212 has a 1×N optical splitter 217 whichreceives the combined signals on the fiber 220. The splitter 217 dividesthe combined signals into N signal paths. Each path has a fiberoptictunable filter 218 which filters out most of the signals from thesplitter 217. Only the signals at a desired wavelength are passed by thefilter 218 to an optoelectronic receiver 219 which translates thefiltered optical signals to electronic signals for the particularreceiver.

Advanced WDM Transmitter System

The transmitter system of the network includes the light sources whoselight signals are combined for transmission over the optical fiber 220to the desired receivers 219. Each of the individual sources has a laserdiode 214 which generates the light signals for a source and an opticalisolator 215 to protect the laser diode from reflections or errantsignals. Such reflections and errant signals may deleteriously affectthe performance of the laser diode.

In the consideration of a fiberoptic network, cost and performance arealways considered. The cost of a single optical isolator may not besignificant, but the total cost of many isolators can become significantin the context of a transmitter system of a network. Furthermore, theperformance of the optical isolators cannot be ignored. If performanceis degraded at the expense of cost, the operation of the network isadversely affected.

The present invention solves or substantially mitigates these problemswith a transmitter system with couplers integrated with the opticalisolator illustrated by FIG. 8 and 10. These devices perform at a highlevel and reduce the number of optical isolators from that of aconventional transmitter system with no degradation in performance.

The present invention replaces the prior art optical isolators 215 inFIG. 19 with high-performance integrated couplers 225, like thoseillustrated by FIG. 10. As shown in FIG. 20, these couplers 225 areconnected to two laser diodes 14 through two input optical fibers,instead of one input optical fiber from a laser diode 214 (FIG. 19).Each coupler 225, integrated with an optical isolator core, is connectedto a combiner 226, which has a reduced number of input terminals,specifically N/2, instead of N.

FIG. 21 shows another transmitter system according to the presentinvention in which the prior art isolators 215 in FIG. 19 are replacedby couplers 224, like those illustrated in FIG. 8. Like the couplers 225in FIG. 3, the couplers 224 in FIG. 21 are connected to two laser diodes214 through two input optical fibers. Each multi-port isolator coupler224 is integrated with an optical isolator and has two output fibers forconnection to the N×1 optical combiner 216 in FIG. 19.

Advanced WDM Receiver System

The prior art receiver system of the network in FIG. 19 can also beimproved according to the present invention. The receiver systemincludes the splitter 217 which receives the combined signals from theoptical fiber 220 and sends a portion of the combined signals to theindividual receivers. Each of the receivers filters out all of thesignals except that from the desired source. This traditional "tree"structure of a receiver system has some inherent problems. One problemis the strength of the optical signals when they reach the individualreceiver. The signals suffer an insertion loss from passing through thesplitter 217 and an inherent loss from the division of the signals intothe "branches" of the receiver system to each receiver. In an N receiversystem, the signals are inherently reduced to 1/N at each receiver.

Another problem is the uniformity in the signal strength in the branchesof the tree. The splitter may not distribute evenly the signals from theoptical fiber. Thus one receiver may operate on much weaker signalscompared to another receiver.

The present invention solves or substantially mitigates these problemswith receiver systems with couplers integrated with wave-lengthdependent filters as illustrated in FIG. 11. The present inventionreplaces the prior art 1×N splitter 217 and filters 218 in FIG. 19 withhigh-performance, integrated WDM filtering couplers. The replacing WDMcouplers have long-pass filters such that light signals of onewavelength travel on one fiber from a coupler and signals of anotherwavelength travel on another fiber from the coupler. The couplers can bearranged in receiver systems with low insertion losses and/oruniformity.

FIG. 22 shows a receiver system using the WDM couplers of FIG. 11. TheWDM couplers 261-167 are cascaded. For the purposes of explanation, thenetwork is assumed to have eight sources at eight different andincreasing wavelengths, λ₁ to λ₈. A first coupler 261 is connected tothe optical fiber linking the transmission system of the network to thereceiver system and receives all of the combined signals, λ₁ to λ₈. Thelong-pass filter of the coupler 261 is selected such that all but theshortest wavelength signal, λ₁, passes to the next coupler 262. The λ₁signal is sent to the individual receiver.

The second coupler 262 passes all but the second shortest wavelengthsignal, λ₂, to the next coupler 263. The coupler 62 reflects the λ₂signal to another individual receiver. In this arrangement each cascadedcoupler 261-267 has its long-pass filter selected to reflect the signalat the next increasing wavelength, λ₁ to λ₈. The final coupler 267passes the signal at the longest wavelength λ₈ to the eighth individualreceiver and reflects the signal λ₇ to the seventh receiver.

FIG. 23, a graph which plots the reflectivity of n-1 couplers versusincreasing wavelength to separate n signals at wavelengths λ₁ to λ_(n),illustrates how the long-pass filters of the couplers in FIG. 22operate.

FIG. 25 is another receiver system using the WDM of FIG. 11. Thecouplers here are in a hybrid arrangement, neither a tree nor cascadeconnection. As done previously, the network is assumed to have eightsources at eight different and increasing wavelengths, λ₁ -λ₈.

A first coupler 271 is connected to the optical fiber linking thetransmission system of the network to the receiver system and receivesall of the combined signals, λ₁ -λ₈. The long-pass filter of the coupler271 is selected such that half of the optical signals at the longerwavelengths, λ₅ -λ₈, pass and half of the optical signals at the shorterwavelengths, λ₁ -λ₄, are reflected. The reflected signals are sent to acoupler 272 which has its long-pass filter set to pass half the signalsat the longer wavelengths, λ₃ -λ₄, and reflect half the signals at theshorter wavelengths, λ₁ -λ₂. The signals reflected by the coupler 272are received by a coupler 274 which has its long-pass filter set to passthe signals at the longer wavelength, λ₂, and to reflect the signals atthe shorter wavelength, λ₁. The signals passed by the coupler 272 arereceived by a coupler 275 which has its long-pass filter set to pass thesignal at the longer wavelength, λ₄, and reflect the signals at theshorter wavelength, λ₃.

Similarly the signals at wavelengths λ₅ -λ₈ passed by the coupler 71 areseparated into signals at individual wavelengths λ₅,λ₆,λ₇ and λ₈ bycouplers 273, 276 and 277. The long-pass filters of these couplers 273,276 and 277 are set to appropriately pass and reflect the input opticalsignals at the different wavelengths.

The table below compares the performances between the tree couplerarrangement in the prior art, and the cascade and hybrid arrangements ofthe present invention for eight receivers. The parameters of the priorart fused tree coupler arrangement are determined from empirical data.The parameters for the cascade and hybrid arrangements of the presentinvention are determined from calculations from the empirical data ofthe reflected insertion loss, ILr, and transmitted insertion loss, ILt,of the WDM coupler of FIG. 24. The maximum insertion loss is a measureof the decline of the weakest signal which reaches one of the receiversof the network from the transmission optical fiber. The uniformity is ameasure of the variation in signal strengths received by the receiversattached to the receiving system.

    ______________________________________    Comparison Table              1 × 8 Cascaded                          1 × 8 Hybrid                                      1 × 8 Fused              Filter-WDM  Filter-WDM  Tree Coupler    Configuration              (Conf. of   (Conf. of   (Conf. of    Parameter FIG. 22)    FIG. 25)    FIG. 19)    ______________________________________    Max. Insertion              4.2         1.8         10.0    Loss (dB)    Uniformity              3.0         0.3         1.2    (dB)    ______________________________________

One observation from the table above is that the hybrid arrangement issuperior to the prior art arrangement in both maximum insertion loss andin uniformity. Another is that the cascade arrangement has a smallermaximum insertion loss than the tree coupler configuration.

Other arrangements may be made by combinations of the cascade and hybridarrangements. For example, the light signals from the transmittingsystem can be split into multiple branches according to the hybridarrangement. The long-pass filter of the splitting coupler is selectedto divide half the incoming signals into each branch. Each branch isthen arranged in a cascade.

While the above is a complete description of the preferred embodimentsof the present invention, various alternatives, modifications andequivalents may be used. It should be evident that the present inventionis equally applicable by making appropriate modifications to theembodiment described above. For example, it should be evident from thecouplers described above that more input and output fibers may beconnected together according to the teachings of the present invention.Therefore, the above description should not be taken as limiting thescope of invention which is defined by the metes and bounds of theappended claims.

What is claimed is:
 1. A coupler for coupling two input optical fibersto one output optical fiber, said coupler comprisinga first sleevehaving an end face, a longitudinal axis and an aperture parallel to saidlongitudinal axis and through said end face, said aperture matching endsections of said two input optical fibers so that said end sections aresnugly held in said aperture, said first sleeve end face coplanar withends of said two input optical fibers and angled with respect to a planeperpendicular to said longitudinal axis; a first GRIN lens in front ofsaid first sleeve end face, said first GRIN lens having an end facedisplaced from, and in close proximity to, said first sleeve end faceand reciprocally angled thereto; a second sleeve having an end face, alongitudinal axis and an aperture parallel to said longitudinal axis andthrough said end face, said aperture matching an end section of saidoutput optical fiber so that said end section is snugly held in saidaperture, said second sleeve end face facing said first sleeve end face,said second sleeve end face coplanar with an end of said output opticalfiber and angled with respect to a plane perpendicular to saidlongitudinal axis; a second GRIN lens in front of said second sleeve endface, said second GRIN lens having an end face displaced from, and inclose proximity to, said second sleeve end face and reciprocally angledthereto; said first and second GRIN lenses having an overall pitch ofnearly 0.5; and said first sleeve, said first GRIN lens, said secondsleeve, and second GRIN lens aligned with respect to each other suchthat said second GRIN lens focusses light signals from each of saidinput optical fibers into said output optical fiber.
 2. The coupler ofclaim 1 further comprising anti-reflection coating, said anti-reflectioncoating covering said end faces of said first sleeve, first GRIN lens,said second sleeve and said second GRIN lens.
 3. The coupler of claim 2wherein said first and second GRIN lenses further comprise second endfaces facing each other and wherein said anti-reflection coating coversboth of said second end faces of said first and second GRIN lenses. 4.The coupler of claim 1 wherein each end section of said two inputoptical fibers comprises a core and an untapered cladding surroundingsaid core.
 5. The coupler of claim 1 wherein said end section of saidoutput optical fiber comprises a core and an untapered claddingsurrounding said core.
 6. The coupler of claim 1 further comprisinganumber of additional input optical fibers, each of said number ofadditional input optical fibers having ends, said first sleeve apertureadapted so that said end sections of said two input optical fibers andsaid additional input optical fibers are held snugly in said aperture,said ends of said two input optical fibers and said additional inputoptical fibers in close proximity with each other and said end face ofsaid first GRIN lens, said ends of additional input optical fibersarranged so that light from one of said additional input optical fiberspasses into said output optical fiber.
 7. The coupler of claim 1 whereinsaid output optical fiber comprises an enlarged core optical fiber. 8.The coupler of claim 7 wherein said enlarged core optical fiber corecomprises a thermally enlarged core optical fiber.
 9. The coupler ofclaim 1 wherein said first and second GRIN lenses have a 0.23 pitch.